MicroElectroMechanical Systems, or MEMS, are an enabling technology. Generally speaking, MEMS devices are integrated circuits containing tiny mechanical, optical, magnetic, electrical, chemical, biological, or other, transducers or actuators. They are manufactured using high-volume silicon wafer fabrication techniques developed over the last 50 years for the microelectronics industry. Their resulting small size and low cost make them attractive for use in an increasing number of applications in consumer, automotive, medical, aerospace/defense, green energy, industrial, and other markets.
In general a MEMS device must interact with a particular aspect of its environment while being protected from damage by the environment. For example, a micro mirror must interact with an electrical addressing signal and with light while being protected from moisture and mechanical damage. An accelerometer must be free to move in response to accelerated motion, but must be protected from dirt and moisture, and perhaps must also be kept under vacuum or low pressure to minimize air damping. In almost every application, an electrical connection must be made between the MEMS transducer or actuator and an external integrated circuit (IC) or printed circuit board (PCB) in order to read the transducer signal or to address the actuator.
Consequently, much effort has gone into developing methods of packaging MEMS devices to protect them while providing access to electrical signals. Initially MEMS packaging consisted of attaching a bare MEMS silicon chip to the base of a metal or ceramic package with adhesive, wire bonding the MEMS to the package leads, and finally attaching a lid to the package. This labor intensive and materially demanding chip-level packaging may account for up to 50-80% of the final packaged device's cost. In many cases, the packaged MEMS still needs to be electrically connected to the IC sensing electronics on a board. As technology improved, the MEMS and IC were integrated into a single package. However, chip placement, wire bonding, and package sealing still led to high device cost.
Numerous subsequent improvements in MEMS packaging have been made to simplify the package and reduce cost. Most of these approaches take advantage of the 2D planar nature of silicon microelectronics fabrication. All microelectronic ICs and most MEMS devices are fabricated by successively depositing thin films such as silicon dioxide, silicon nitride, polycrystalline silicon, metals, etc., using a photolithographic process to form the desired 2D shape of the film (e.g. transistor gate, MEMS accelerometer proof mass, etc.), and to etch a pattern into the film. In some cases, the photolithographic process produces a form into which the film is plated or deposited to form the desired pattern. This process is repeated over and over to form the final device. As a result, most MEMS devices are planar or two-dimensional since they consist of a stack of very thin films, each typically on the order of micrometers thick or less.
Typically, a cap (e.g. silicon or glass) is placed over the MEMS to protect it, and electrical contact is made to the top of the MEMS. Most of these integration approaches are based on a 2D architecture with sensor detection and signal transduction in the plane of the device. For example, almost all accelerometers and gyroscopes use comb capacitors for drive and detection in the plane of the device. The silicon below the MEMS and in the cap above it is non-functional. However, many MEMS applications are inherently 3 dimensional. For example some microfluidics chips need to route fluids in three dimensions. Accelerometer and gyroscope designs benefit from proof masses which have low Brownian noise and can move in three dimensions. In these cases, it is desirable to have sensor electrodes or actuators located above, beneath, within, or around the MEMS elements. These electrodes are not easily accessible using the typical 2D planar architectures described above. Consequently, 3D devices have been largely constrained to packaging architectures that use non-functional package caps.
What is needed is a MEMS device which allows transmitting electrical signals from within the device to at least one cap, while enclosing the MEMS sensing element(s). It would also be desirable to provide a cost-effective manufacturing method for such device.